Life and Death of Larger stars

The life of a Small Star is very long, and relatively peaceful. Since they don’t have enough mass to go the whole way in fusing atoms, smaller stars usually stop fusion after Magnesium is formed. Then, they quietly shed away their outer layers. Soon, all we are left with is the core of the star, a White Dwarf. I explained well the life and death of small stars in Previous Article.

But all this is boring. Let’s go to the part where things are violent. Stars much heavier than the sun live more violently, and die in a similar fashion as well.

Let’s Begin The Journey

Heavier stars use of their fuel way too fast, and their life is usually not more than 100 million years. Initially, hydrogen is fused into helium, as expected. However, they fuse a lot more hydrogen than the Sun. Consequently, they are much hotter, and brighter than the sun.

When the hydrogen runs out, fusion of helium begins. This is also similar to lighter stars on an atomic level. However, on the larger scale, drastic changes are happening. Such a star is so large that it has a seperate term for it – a Supergiant. They can span the entire visible spectrum, with blue supergiants being the hottest and red the coolest. This is the shortest phase of the life of the star.

A blue supergiant as compared to the solar system.

End Of Life

The extreme gravitational forces mean that the fusion keeps going beyond magnesium, into the d-block elements. Nuclear fusion goes on as usual. However, the blockade comes when iron is formed in the core. And iron ends up being the death of the star.

Nuclear fusion releases energy which counterbalances gravity. The problem is, fusion of iron is endothermic – it does not give energy, it takes it. This is troublesome, as there is no source of energy now for the star to fight against that gravitational pull. Gravity begins to compress all the matter inwards until the core gets to heated up, that it explodes. This is one of the most energetic cosmic events, called a Supernova. A single Supernova can release as much energy as the Sun will do in its lifetime. In a short span, due to the large amounts of energy released, iron fuses to form a wide variety of heavier elements, including gold, silver and uranium.

After Then?

Supernova explosions throw out a large majority of the mass of the star out. In the end, once again, only the core of the star remains. But this time, it is no ordinary core.

Because of the large mass of the core, the quantum degeneracy pressure cannot combat with the inward pull. In the end, it comes down to the mass of the remnant core.

If the mass of the core is less than 3 times the mass of the sun, the core collapses into a Neutron star. It is the second most bizarre thing we have ever noticed. Protons and electrons are squished so tightly they combine into neutrons. This releases huge amounts of Energy and Neutrinos.

Neutron Stars

The neutrons are very stable in this configuration, and repel each other due to the Neutron degeneracy pressure.

A neutron star is the densest physical object in the world.

A single spoon of material from such a star would weigh more than the entire Empire State Building. Neutron stars can exist in this phase for billions of years. Neutrons stars are very active electromagnetic objects.

Neutron stars also spin about their axis, and have a very powerful magnetic field. This is due to the neutron spin adding up in the star. Such stars can rotate about their axis 1000 times every second, and give off very strong electromagnetic radiation. We identify these stars as Pulsars.

The mass of star expelled after the supernova is rich in many elements. Such nebulae are the birthplaces for other stars as well. In fact, it was the same case for the entire Solar System as well.

Every single atom present on the Earth was once a part of a star, thrown away in a violent bang. We are literally made of stardust.

However, if the mass of the star is more than 3 times the mass of the Sun, then nothing can prevent the collapse. Even the neutron degeneracy is not enough to overcome gravity. All the mass of the star gets compressed into a single point of infinite density and curvature, called a Singularity. The space surrounding the singularity is a Black Hole. Black holes mess up the geometry of Space-Time itself. Their gravitational pull is so strong that even light cannot escape it. Thus, the name Black Hole.

Black holes are not as simple to understand as the other phases of a star. Physical laws break down in the vicinity of the singularity. Black holes were initially just a mathematical prediction of Einstein’s General Theory of Relativity.

In fact, we should have a seperate discussion on the weirdness of these gravitational monsters.